1. Field of the Invention
The present invention relates generally to a method for fabricating a silicon carbide substrate, and more specifically, to a method of forming a beta silicon carbide layer on silicon dioxide which is formed on a silicon substrate.
2. Description of the Related Art
Silicon carbide (SiC) is known as a semiconductor material which has a wide band gap in its energy level compared to that of silicon, and has a capability for fabricating semiconductor devices which can be used in high temperature environments. Especially, beta silicon carbide (.beta.-SiC) is known to be desirable for fabricating various active devices, such as transistors or field effect transistors (FETs). More detail of those devices fabricated in SiC can be seen in, for example, "Experimental 3C-SiC MOSFET" by Y. Kondo et al. IEEE ED Letters Vol. EDL-7. No. 7, July 1986.
Single crystals of .beta.-SiC usable for fabricating active devices such as transistors or FETs are still not available by the present state-of-the-art semiconductor technology. Therefore, these devices are fabricated using an epitaxially grown silicon carbide layer. It is considered that a better epitaxial layer of SiC may be obtained using a SiC substrate, but since SiC substrate which is large enough for fabricating various devices is not available, the growth of SiC layer is usually done using a silicon substrate. At the present time, therefore, a silicon substrate is considered to be the only practical semiconductor substrate on which .beta.-SiC can be epitaxially grown. Such crystal growth is called hetero-epitaxial growth, because the crystal of the substrate and the grown crystal are different from each other.
An exemplary structure of a FET formed in a SiC epitaxial layer is shown in FIG. 1(a). In the figure, 101 designates a silicon substrate, and 102 is a hetero-epitaxially grown .beta.-SiC layer. In this example, both the silicon substrate and the SiC layer have n type conductivity. A source region 103, and drain region 104 are fabricated by doping p type impurities. Over the channel region is formed a gate oxide layer 105 which is composed of a silicon dioxide (SiO.sub.2), and over which a gate electrode 106 is formed. A surface of the device is coated with a silicon dioxide layer 107. The electrodes indicated by G, S and D are respectively the gate, source and drain electrodes which are connected to respective regions through contact holes formed in the SiO.sub.2 layer 107.
In the above structure of a FET, it was found that leakage current is relatively large. This is mainly due to the fact that the SiC epitaxial layer 102 formed on the silicon substrate 101 is poor in quality. Therefore, the pn junction formed in such SiC layer is apt to become a leaky junction Moreover, the conductivity of the silicon substrate 101 becomes higher than that of the SiC layer 102 at high temperatures (500-600 .degree. C. for example), in which the SiC devices are expected to operate. Therefore, FETs such as the one shown in FIG. 1(a) are difficult to be actually used in high temperature environments. Such defects occur on other types of devices made from SiC material.
To eliminate these defects of SiC devices, it has been proposed to insert an additional layer between the SiC layer and the Si substrate in order to suppress the leakage current through the silicon substrate. For example, in "Fabrication of Inversion-Type n-Channel MOSFET's Using Cubic-SiC on Si(100)" by K. Shibahara et al., IEEE ELECTRON DEVICE LETTERS, Vol. EDL-7, No. 12, December 1986, or "Insulated-Gate and Junction-Gate FET's of CVD-Grown .beta.-SiC" by K. Furukawa et al. IEEE ED Letters, Vol. EDL-8, No. 2, Feb. 1987, it has been proposed to insert another conductivity type SiC layer and to form a pn junction between the former SiC layer. This pn junction prevents the leakage current.
A known type of silicon device is a SOI (silicon on insulator) structure, in which a silicon crystal is grown on an insulator such as silicon dioxide layer formed on silicon substrate. If it is possible to grow a .beta.-SiC crystal on an insulator, it must be effective for preventing the leakage current through the substrate.
A fundamental configuration of a SOI is shown in FIG. 1(b). The same reference numerals designate the same parts as in FIGS. 1(a) and 1(b). Compared to FIG. 1(a), the proposed new device of FIG. 1(b) includes a silicon dioxide layer 108 between the silicon substrate 101 and the SiC layer 102. By this silicon dioxide layer 108, the leakage current of the SiC FET will be reduced to a great extent, because the leak current running through the silicon substrate 101 is suppressed.
A problem is now how to fabricate such a device, that is, an SOI type device formed by SiC. For silicon devices, the SOI structure that means "semiconductor on insulator" has already been realized (SOI sometimes means silicon on insulator structure). With regard to SiC devices, however, a method to provide the SOI (SiC on insulator) structure has not heretofore been established. For making such devices, a substrate having a structure of SiC on insulator is required. One proposal is using a .alpha.-SiC crystal as the insulating substrate. The resistivity of undoped .alpha.-SiC is very high, but a large size perfect crystal of .alpha.-siC is still not available. The size of the substrate is another important factor for fabricating various semiconductor devices.